Add functionalities for optical tactile plugin

[mcres]

This PR is a change of target from this PR in my fork.

This PR adds the following:

• Service for enabling/disabling the plugin
• Topic for the normal forces computed
• Test
• Visualize the contacts from the contact sensor

[mcres]

CC @mabelzhang

[mabelzhang]

Will review after #229 is merged (it's very close), then will update feature branch to reduce the diffs here.

[chapulina]

#229 is in, moving this to "In Review"!

[chapulina]

then will update feature branch to reduce the diffs here.

I just noticed that this PR isn't targeting ign-gazebo4. Can it be retargeted and rebased, @mabelzhang ?

[mabelzhang]

I reviewed most of the files and pushed some changes, mostly for style.
Only file I still need to review is the tests.

[mabelzhang]

I'm saving the Mac CI error message from the new test, in case the link dies before I come back to fix it.
Might need to add some directives for Mac.

149: [ RUN      ] OpticalTactilePluginTest.ForcesOnPlane
149: [Err] [Ogre2RenderEngine.cc:436] Unable to find Ogre Plugin[/usr/local/opt/ogre2.1/lib/OGRE-2.1/RenderSystem_GL3Plus]. Rendering will not be possible.Make sure you have installed OGRE properly.
149: [Err] [Ogre2RenderEngine.cc:739]  Unable to create the rendering window

[chapulina]

I pushed some tweaks in 50f4250. This is working well for me! We just need to check the macOS tests. They're currently failing for a different reason, which will be fixed by #792 .

[chapulina]

The macOS tests are failing to load Ogre just like in gazebosim/gz-sensors#66 and gazebosim/gz-gui#145, so I disabled the test for macOS on 4ffad4d

[mabelzhang]

Approving because Mac failing tests were my only concern

[chapulina]

Good to go! 🚀